Eccentric plug valves have been in use for a long time. U.S. Pat. No. 2,803,426 to De Zurik is typical of this class of valves. In such a valve, the valve chamber is cylindrical. The inlet to the valve chamber and the outlet from the valve chamber are transverse to the axis of the valve chamber. The valve plug is mounted for rotation in the valve chamber about an axis parallel to, but slightly offset from, the axis of the valve chamber. The offset of the axis of the valve plug from the axis of the valve chamber gives the eccentric plug its name.
The cylindrical sealing face on the valve plug mates with the seating surface of the valve chamber at the periphery of the inlet leading into the valve chamber to close the valve. The cooperation of the seating surface with the offset plug sealing face provides the advantageous feature of eccentric plug valves, namely, that as the plug rotates to seal the inlet, there is virtually no sliding contact between the seat in the valve chamber and the face on the plug.
In the most popular eccentric plug valves which are commercially available now, the sealing is effected by a rubber-encapsulated plug mating with a nickel seat ring welded into the wall of the valve chamber around the inlet to the valve chamber. When the rubber sealing surface of the plug wears, as may occur from the passage of solid substances and fluids through the valve when it is opened and from repeated opening and closing of the valve, the entire plug is replaced. This is relatively costly.